Electroactive polymers can generally be switched between two or more stable oxidation states, giving rise to changes in properties including conductivity, color, volume, and transmissivity. [G. Inzelt, M. Pineri, J. W. Schultze, and M. A. Vorotyntsev, Electrochim. Acta, 45, 2403 (2000)]. Electroactive polymers which have been oxidized from a neutral state are said to be p-doped, by analogy to semiconductor terminology. Likewise, polymers that have been reduced from a neutral state are said to be n-doped. Owing to the inherent stability of carbocations, p-dopable materials are quite well known and have been thoroughly documented. [G. Inzelt, M. Pineri, J. W. Schultze, and M. A. Vorotyntsev, Electrochim. Acta, 45, 2403 (2000); J. Jagur-Grodzinski, Polym. Adv. Tech., 13, 615 (2002); and, J. W. Schultze and H. Karabulut, Electrochim. Acta, 50 1739 (2005)]. However, stable n-doped polymers have heretofore been unreported. [D. M. de Leeuw, M. M. J. Simenon, A. R. Brown, and R. E. F. Einerhand, Synth. Met., 87, 53 (1997); K. Wilbourn and R. W. Murray, Macromolecules, 21, 89 (1988); and, M. Quinto, S. A. Jenekhe, and A. J. Bard, Chem. Mater. 13, 2824 (2001)]. Such n-doped polymers would be desirable for the same reasons that p-doped polymers have been desired and prepared, as well as for use in applications such as batteries and supercapacitors, for example. [A. Rudge, J. Davey, I. Raistrick, S. Gottesfeld, and J. P. Ferraris, J. Power Sources, 47, 89 (1994)]. The instability of n-doping conjugated polymers is most likely due to the highly reactive nature of carbanions. [D. M. de Leeuw, M. M. J. Simenon, A. R. Brown, and R. E. F. Einerhand, Synth. Met., 87, 53 (1997)].
One approach being explored to obtain stable n-doping polymers is the synthesis of donor-acceptor materials. [A. Berlin, G. Zotti, S. Zecchin, G. Schiavon, B. Vercelli, and A. Zanelli, Chem. Mater., 16, 3667 (2004); D. J. Irvin, C. J. DuBois, and J. R. Reynolds, Chem. Comm. 2121 (1999); P. J. Skabara, I. M. Serebryakov, I. F. Perepichka, N. S. Sariciftci, H. Neugebauer, and A. Cravino, Macromolecules, 34, 2232 (2001); and, H-F. Lu, H. S. O. Chan, and S-C. Ng, Macromolecules, 36, 1543 (2003)]. In a donor-acceptor type of system, the polymer HOMO (highest occupied molecular orbital) is energetically similar to the relatively high-energy HOMO of the donor material, while the polymer LUMO (lowest unoccupied molecular orbital) is energetically similar to the relatively low-energy LUMO of the acceptor. This type of electronic architecture leads to a small HOMO-LUMO gap in the polymers and consequently to a low-lying polymer LUMO suitable for accepting charge.
The electron-poor functionality of the acceptor groups can be obtained in at least two ways. In the most common approach, electron-withdrawing substituents such as nitro- or fluoro-groups, for example, are incorporated pendant to the main chain of the polymer. [D. J. Irvin, C. J. DuBois, and J. R. Reynolds, Chem Comm. 2121 (1999); and, P. J. Skabara, I. M. Serebryakov, I. F. Perepichka, N. S. Sariciftci, H. Neugebauer, and A. Cravino, Macromolecules, 34, 2232 (2001)]. While this method can yield electron-deficient monomer units and ultimately electron-deficient polymers, it is likely that the substituents act as charge traps, hindering electron mobility.
Electron mobility might be improved, without the aid of pendant groups, by incorporation of functional groups that are themselves intrinsically electron-deficient such as the high nitrogen heterocycles. Typically, as the number of imine-type nitrogens replacing carbon in a given aromatic ring increases, so too does that ring's electron affinity. [G. Brocks and A. Tol, Synth. Met., 101, 516 (1999)]. Empirically, the higher the electron affinity of the polymer, the more stable the polymer will be in the n-doped state. [A. P. Kulkarni, C. J. Tonzola, A. Babel, and S. A. Jenekhe, Chem. Mater. 16, 4556, (2004)]. Incorporation of these high nitrogen heterocycles into conjugated polymers should result in n-dopable polymers with good electron mobility. Limited research into this type of donor-acceptor polymer has been conducted by others. [A. Berlin, G. Zotti, S. Zecchin, G. Schiavon, B. Vercelli, and A. Zanelli, Chem. Mater., 16, 3667 (2004); and, H-F. Lu, H. S. O. Chan, and S-C. Ng, Macromolecules, 36, 1543 (2003)]. The present invention discloses a new stable n-doping donor-acceptor polymer.